This invention deals with recovery of heat values from biomass material such as wet wood waste, peat or the like. Of particular interest is the recovery of energy from waste generated by the forest products industries, commonly called "hog fuel".
As fossil fuel costs have escalated, operators processing wood as a raw material, especially in sawmills, pulp mills and composite wood products operations, have become more interested in recovering the heat energy value of wood wastes that are otherwise unsuitable for conversion into salable products. Many facilities generate a sufficient amount of such waste to meet significant portions of their energy requirements. Others have access to supplies of other biomass materials such as peat or whole tree chips which, if suitable methods of heating value recovery were available, could constitute a low cost replacement for fuel oil or natural gas.
Wood wastes from sawmilling and related raw wood handling operations have a number of characteristics that make efficient recovery of heating values difficult. The waste is usually wet, often in excess of 50 percent moisture by weight. Each mill source of waste has its own characteristic composition and moisture content. Much sawmill and pulp mill waste is accumulated and stored out in the weather where it soaks up rainwater during wet periods of the year.
A second problem with wood waste is that it varies greatly in size. These wastes are generated from every wood handling and processing operation. The wastes range from sander dust of 0.1-3 mm particle size to log yard debris which may exceed dimensions of 200 mm in diameter and be over a meter in length. The wood waste is often comminuted or "hogged" to break up oversize material by means of a hammer mill, producing wood waste or "hog fuel" having a particle size of less than 100 mm. Secondary hogging may further reduce size to less than 25 mm. The waste from a sawmill will contain a fair percentage of much smaller particles originating as sawdust.
A common practice in the past has been to burn wet hog fuels on a grate in a combination oil-wood waste boiler. The supplemental oil permits the boiler to more easily follow process demand variations for steam, and sustains combustion in the situation when the hog fuel is very wet. Finer portions of the hog fuel waste may be burned in air suspension, as by means of a swirl stabilized burner of the type described by Michelfelder et al. in U.S. Pat. No. 4,333,405.
Recent techniques of heat recovery from hog fuel require a reduction in moisture content of the hog fuel before it is fed to the boiler. Studies show that reducing the initial moisture content of the fuel improves steam production and reduces boiler stack emissions. A state-of-the-art process that dries a portion of the fuel prior to burning and burns a substantial amount in air suspension is described by Spurrell in U.S. Pat. No. 4,235,174. In this process, the largest size material from the hog fuel pile is burned in a fluid bed burner. The combustion products from the fluid bed burner are then used to dry the balance of the hog fuel in a rotary dryer before it is fed into a combination oil-wood waste boiler. The dried fuel is separated by size. The coarser fraction, up to about 100 mm in maximum dimension and at about 35% moisture, burns on a furnace grate while a fines fraction, at about 15% moisture and a particle size of less than 1/8 inch (3.2 mm) diameter, is injected in air suspension into the boiler.
The Spurrell process, however, requires an oil pilot on the injected fines portion of the fuel in order to sustain stable combustion. This oil pilot represents a substantial use of fossil fuel, up to 30% of the total burner rating at full burner loads.
Certain wood wastes have in the past been recognized as burnable in furnaces without a grate or supplementary oil support. For example, sander dust which is of very fine particle size distribution and about 5% moisture content has been burned successfully in air suspension. Schwieger, in an informative survey article entitled "Power from Wood", Power, 124 (2): S1-S32 (1980), describes sander dust, at about 12% moisture, as being fired to a package boiler. The top size of this material is said to be about 800 .mu.m. Even so an oil pilot is recommended, suggesting unstable combustion conditions.
Very fine materials such as sander dust, however, generally constitute only a very minor portion of the waste available at the typical wood processing mill, particularly those integrated with pulp production facilities. The amounts of these dry, fine wood wastes at most facilities are usually not sufficient to meet a significant percentage of the energy requirements of the typical mill. However, at many facilities generating wood wastes the hog fuel pile as a whole has this capability.
Certain larger size and higher moisture ranges of wood material, even when the wood waste is over 60% moisture content, can be burned without oil support where combustion is carried out in refractory lined furnaces or kilns. In a refractory furnace the firebox is lined with ceramic which attains a temperature of roughly 800.degree. C. (1500.degree. F.) or higher. The hot gases then contact the steam generating tubes. The heat retained by the mass of ceramic is continually reradiated to help sustain stable combustion in the fire box, permitting otherwise difficult-to-burn materials or wastes to be burned without oil support. Refractory furnaces have a high initial cost and the effects of high firebox temperatures result in high maintenance costs. They also normally require a larger boiler tube surface area since the tubes have poor exposure to the hottest part of the flame.
In a hog fuel burning system described by Baardson in U.S. Pat. No. 3,831,535, wood waste is dried and pulverized to a maximum particle size of 5/16" (7.9 mm). This material is accumulated in a bin and injected for combusion in a refractory lined chamber where radiation from the refractory provides support for stabilized combustion.
Because of lower capital costs for construction and lower maintenance costs, industry favors the use of "water-wall" boilers wherein the flame is substantially surrounded by water tubes which generally reach only about 315.degree. C. (600.degree. F.). In these boiler configurations, the walls are relatively cold compared to the flame and are more efficient heat absorbers. The absence of hot firebox refractory surfaces reduces the amount of radiation support available to help sustain the ignition process. As a result, suspension firing of water-wall boilers with conventionally available hog fuels has generally required the use of a fossil fuel pilot to continually provide energy to raise the fuel to ignition temperature.
A more recent approach to burning the larger fraction of the hog fuel pile has involved pulverizing the hog fuel to a smaller particle size range than that of Baardson. Eneroth et al. in U.S. Pat. No. 4,229,183, teach improved hog fuel burning by simultaneously grinding and drying the fuel to 10-15% moisture. The flow from the pulverizer enters a cyclone which separates the fuel from the air flow. The fuel is then re-suspended in air and injected into a boiler. No grate is required. Fagerlund, Tappi 63 (3): 35-36 (1980), further describes the Eneroth method as grinding the wood fuel down to a particle size of 1-3 mm. An oil pilot equivalent to 5% of the burner rating is required for flame control. Fagerlund expresses the hope that control systems will be developed in the future so that no auxiliary oil will be needed.
Rivers et al. in U.S. Pat. No. 4,532,873, commonly assigned with the present application, describe a method used to prepare wet fibrous vegetable matter fuels, including hog fuel, for suspension burning in a water wall heat recovery boiler. This method does not require a gas or fuel oil pilot for stability. However, it does require drying the hog fuel stream and pulverizing it in its totality to a very fine particle size. The resulting fuel is preferably all less than 1 mm with at least 15%, preferably at least 40% less than 150 .mu.m.
Leikert et al, in U.S. Pat. No. 4,436,038 describe a system for burning pulverized coal in an air suspension burner in which a portion of the finest particle size material is separated. While the inventors are somewhat unclear as to the use of this fine material, it appears to serve as fuel for an initial igniting flame or pilot light rather than as a material that is used continuously to sustain the flame.
The use of a continuous oil pilot, even in quantities as small as 5%-10% of the total heat input, can become very expensive. As an example, assuming a 10% fuel oil requirement, a suspension fired boiler producing 225,000 kg/hr of steam would consume 10-12 bbl of oil/hr or approximately 100,000 bbl/year. At a price of $26/bbl this approaches $2.5 million/year for supplemental fuel.
Pulverizing the entire fuel stream to fine particle sizes, as described by Rivers et al., is expensive in terms of capital, maintenance and operating costs. Wood and other fibrous biomass material are inherently much more difficult to pulverize that coal, for example. For a hog fuel source consisting of about 75% wood fiber and 25% bark, if this is dried to 15% average moisture and presized to pass through a 6.4 mm (1/4 in) screen, grinding energy to produce a product containing 40% less than 100 .mu.m in size is about 95 kW.h/t. By way of comparison, coal precrushed to about 0.5 mm only requires 11-21 kW.h/t to produce a product with 80% passing a 74 .mu.m (200 Mesh Tyler series) screen. The exact power required will depend on coal rank and the particular equipment chosen.
Bark is known to be easier to comminute than wood. If the hog fuel source noted above was composed of 25% wood fiber and 75% bark, with all other parameters being similar, grinding energy would be about 28 kW.h/t. This is about 1.8 times greater than coal if 16 kW.h/t is used as an average figure for the latter material.
With a 75% wood fiber and 25% bark hog fuel source, using the process described by Rivers et al., for a 2.25.times.10.sup.5 kg/hr power boiler, annual grinding energy consumption would be about 1.3.times.10.sup.8 MJ. At the optimistic power rate of $0.011/MJ the annual power cost for fuel grinding alone would be about $1.4 million/year.
Mill experience clearly indicates that pulverizer reliability is a serious problem. High-speed impact mills are the only practical machines capable of continuously pulverizing the required amounts of woody types of biomass used in a Rivers-type process. Past experience using this type of mill with hog fuel has shown 10% or more downtime on each pulverizer for maintenance.
With the present state of the art in pulverizing hog fuel, in order to conduct a commercial operation, one must either expend capital to purchase oversize and/or backup pulverizers or burn oil during pulverizer downtime. None of these alternatives are particularly attractive.
Other biomass materials, such as peat for example, are similar to hog fuel in that they are wet and of unsuitable physical form or size. Thus, these potential fuels are generally not utilized in many parts of the world. While the discussion which follows focuses upon wet wood waste or hog fuel, the invention is applicable to any wet biomass matter.
It can be said in summary that the present state of the art for burning biomass materials such as hog fuel in air suspension in a water-wall boiler requires either a stabilizing fossil fuel pilot or drying the biomass material and grinding it in its entirety to a fine particle size.